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HARMONIC & INTERMODULATION DISTORTION Anjani Kumar Yadav 11115013
| Date post: | 20-Jun-2015 |
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HARMONIC & INTERMODULATION DISTORTION
Anjani Kumar Yadav
11115013
WHAT IS DISTORTION IN ELECTRICAL SYSTEMS?
Alteration of the original shape (or other characteristic) of sound or waveform.
WHAT IS HARMONICS? “A COMPONENT FREQUENCY OF A HARMONIC MOTION OF AN ELECTRO MAGNETIC WAVE i.e. AN INTEGRAL MULTIPLE OF FUNDAMENTAL FREQUENCY”.
FUNDAMENTAL FREQUENCY in India is 50Hz.
WHAT CAUSES HARMONICS ? NON-LINEAR LOAD ::: CURRENT IS NOT PROPOTIONAL TO VOLTAGE
e.g . DIODE , TRANSISTOR , NON-LINEAR AMPLIFIERS
ETC.
Some devices and loads:
Converters,
Devices which includes semi-conductor elements,
Generators,
Motors,
Computers,
Electronic ballasts,
Switching power supplies,
Welding machines,
Control circuits,
HVDC transmission systems,
Electrical Communication systems.
Transformers
Total Harmonic Distortion(THD): ------ Measure of amount of harmonics
THD of the load should be less than 7% for acceptable performance
THD for intermodulation distortion can also be calculated in a similar way.
TOTAL HARMONIC DISTORTION (THD)
INTERMODULATION DISTORTION Intermodulation or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies in a system with nonlinearities
Intermodulation occurs when the input to a non-linear system is composed of two or more frequencies. Consider an input signal that contains three frequency components at fa ,fb and fc which may be expressed as :
Output signal
Y(t) = G(x(t))
Y(t) will contain three frequencies fa ,fb and fc of the input signal as well as number of linear combinations of these fundamental frequencies:
Kafa+Kbfb +Kcfc
Where Ka Kb and Kc are arbitrary integers which can assume positive or negative values. These are the intermodulation products (or IMPs).
Intermodulation order:
The order O of a given intermodulation product is the sum of the absolute values of the coefficients
O= |Ka|+|Kb|+|Kc|
In our example 3rd order intermodulation products occur where
The performance of an ideal amplifier can be represented by the transfer function:
An amplifier with some distortion due to nonlinearities can be expressed by a transfer function in the form of a power series expansion:
inout VAAV 10
....43210
432
ininininout VAVAVAVAAV
)cos()cos( 2211 tVtVVin An input signal with two frequencies 1 and 2 may be shown as:
The first order term inVAA 10 gives the fundamental products )cos()cos( 2211110 tVAtVAAVout
determines the second order products:2
2 inVAThe second order term
)]cos()[cos(2
)2cos(2
)2cos(222 2121
2122
222
1
212
222
2122
2 ttttVVA
tVA
tVAVAVA
VA in
DC terms 2nd harmonic terms 2nd order IMD terms
determines the third order products:3
3 inVAThe third order term
)cos(
2cos
23
)cos(22
32
32
221
31
312
2133
3 tV
VVA
tV
VVA
VA in Fundamental frequency terms
3rd harmonic terms
3rd order IMD terms – The troublemakers
)3cos(4
)3cos(4 2
323
1
313 t
VAt
VA
)]2cos()2[cos(4
3)]2cos()2[cos(
4
31212
2213
21212
213 tttt
VVAtttt
VVA
THIRD ORDER INTERCEPT POINT
The 3rd order products will be the largest (loudest) of the intermodulation products.
As a general rule, the 3rd order products will increase (grow) 3-times faster than the fundamental signal (the signal of interest)
DISADVANTAGESIncreased losses on the distribution system due to increase in the effective rms currentOverloads, vibration and premature ageing of the generators, transformers and motors as well as increase in the noise level.Distortion of the supply voltage that can disturb the operation of the sensitive loadsDisturbances in the communication networks and telephone lines
SOLUTIONS Selective filters for low-order harmonics.
Active power filters before non-linear loads.
THD based power tarrifs methods.
REFERENCES :
www.wikipedia.com
http://ieeexplore.ieee.org
http://ecmweb.com
http://www.researchgate.net
